For anyone that has been following my silly little project, I have the first version of the App View live over at soapstone.social. It's been up for a few weeks now to make client development easier so I may as well share with other folks.
Basically you can query for messages near you, and you can write new soapstone records to your PDS and they will be indexed. There's actually a whole big discussion happening on what should be the recommended way to go about writing new records (proxy through PDS vs through App Views) so I'm keeping an eye on that.
I'm not 100% happy with the Lexicon, but it's good enough for now. The largest and most important hurtle is going to be the app development. If I can get an app working the delivers on the experience I want then I'll probably end up making another set of big revisions to the Lexicon and App View implementation.